Update python3 to python3.6
Update python3 to python3.6

It conforms with Alpine 3.8 (It updates the related tox entry).

Change-Id: I7ba8e3cf5bae2f48ff5d802511a288446bca645a
Signed-off-by: Cédric Ollivier <d48310251a4a484d041bc5d09a9ac4d86d20f793@orange.com>

opnfv/functest,opnfv/functest